1. Field of the Invention
The present invention is a process whereby hydrocarbon feed streams comprising mixtures of non-normal, branched paraffins (i.e., isoparaffins) and normal, unbranched, straight chain paraffins (n-paraffins) are separated into a permeate stream enriched in isoparaffins and a retentate stream enriched in n-paraffins as compared to the feed by the process of contacting the hydrocarbon feed with one face of a non-selective, micro-porous partition barrier membrane while simultaneously contacting the opposite face of said micro-porous partition barrier membrane with a polar solvent. Preferably the feed and polar solvent are passed countercurrently across the respective faces of the microporous partition barrier membrane. The isoparaffins in the feed selectively permeate through the micro-porous partition barrier in response to the polar solvent on the solvent side of the membrane. A permeate enriched in isoparaffins and a retentate of decreased isoparaffin (but of increased n-paraffin) content as compared to the feed is obtained.
2. Description of the Related Art
The separation of isoparaffins from n-paraffins is a very desirable processing step in hydrocarbon refining. Removal of n-paraffins from the motor gasoline pool will significantly enhance the total pool octane quality.
Because the isoparaffins and n-paraffins of substantially the same carbon number range possess boiling point in overlapping ranges, separation of isoparaffins from n-paraffins by distillation is difficult. While separation based on melting point may be possible, such a separation is very energy intensive requiring extensive refrigeration to effect the separation.
The use of selective membranes has been suggested. U.S. Pat. No. 2,947,687 and U.S. Pat. No. 3,043,891 disclose the separation of hydrocarbon mixtures by passing across the face of a non-porous membrane through which at least one component of the hydrocarbon mixture will permeate.
U.S. Pat. No. 3,043,891 teaches a process for increasing the permeation rate of saturated hydrocarbons through non-porous membranes which are capable of separating hydrocarbons according to type, and/or molecular configuration, and/or boiling point or molecular weight. The patent teaches that the permeation process is increased by contacting the membrane during the permeation process with an added hydrocarbon solvent for the membrane. This solvent may contact the membrane on the feed side, the permeate side or on both sides. Representative of such permeation accelerating solvents include aromatics and unsaturated hydrocarbons such as olefins or diolefins. The solvent is described as being a solvent for the membrane, i.e., swells the membrane.
The membranes employed are described as non-porous and include natural or synthetic rubber, gum rubber, chloroprene, neoprene, vinyl polymers such as styrene polymer, polyisobutylene, certain cellulose ethers.
The patent indicates that saturated molecules will permeate through the membrane in the following sequence of increasing selectivity: open chain highly branched hydrocarbons, &lt;open chain with lesser degree of branching; &lt;closed chain (e.g., cycloparaffins) and alkyl cycloparaffins, &lt;straight chain or normal paraffins (i.e., straight chain paraffins more readily permeate). Use of the membrane solvent will substantially increase the permeation without substantially altering the selectivity
U.S. Pat. No. 2,947,687 teaches the separation of hydrocarbons by type through a non-porous membrane using a membrane solvent to enhance the permeation rate. Membrane solvents include substituted hydrocarbons which are soluble in, and have solvent power for, the membrane. The hydrocarbon solvent is an organic compound containing one or more atoms of halogen, oxygen, sulfur or nitrogen. Thus, materials such as carbontetrachloride, alcohols, ketones, esters, ethers, carboxylic acids, mercaptans, sulfides (e.g., diethylsulfide etc.), nitropropane, nitrobenzene, acetonitrile, formamide, ethylene diamine, etc. may be employed in an amount ranging from 1 to 100% based on total solvent to hydrocarbon feed. The process may be operated at a pressure differential between the feed and permeate zone with the permeate being removed by vacuum. Alternately the permeate can be removed by a sweep stream such as steam, air, butane, etc.
The membrane is non-porous and includes natural or synthetic rubber, vinyl polymers, cellulose esters, cellulose ethers.
The process can use any hydrocarbon source as feed and the separation achieved is in the order: saturated hydrocarbons, &lt;unsaturated hydrocarbons, &lt;aromatics. Saturated hydrocarbons of approximately the same boiling range permeate in the order of increasing selectively: branched chain, &lt;cyclic-chain, &lt;straight chain configuration, i.e., straight chain paraffins more readily permeate through the membrane.
In an example methyl cyclohexane is separated from an equal volume mixture of methyl cyclohexane and isooctane using 5% methyl ethyl ketone as solvent. An operating pressure differential of 400 mm Hg was maintained and the temperature was 52.degree. C. and 82.degree. C. The methyl cyclohexane preferentially permeated through the membrane.
U.S. Pat. No. 3,956,112 teaches a membrane solvent extraction process. The membrane solvent extraction system is utilized to separate two substantially immiscible liquids and extract a solute through a solvent swollen membrane from one solvent liquid phase to the extracting solvent liquid without direct contact between the liquid phases. The membrane is substantially non-porous. Table III of U.S. Pat. No. 3,956,112 compares the invention of '112 with competing processes. One of these processes is described as direct extraction via porous partition. That process is practiced using two immiscible solvents. The driving force is the chemical potential depending on the partition coefficient of the solute in the two solvents. The process employs a porous membrane or partition wall. In that process solutes from one solvent are transferred to the extraction solvent via direct solvent-solvent contact.
U.S. Pat. No. 3,140,256 teaches a membrane separation process employing a membrane comprised of a cellulose derivative (e.g., cellulose ester or ether) modified by reaction with aldehydes, organic di isocyanate, organic monoisocyanate, organo-phosphorus chlorides and organo-sulfur chlorides. Hydrocarbon feeds can be separated into these components by type using the membrane, e.g., aromatics can be separated from unsaturated hydrocarbon (olefins or di-olefins) and/or from paraffins, or branched chain aliphatic hydrocarbons can be separated from other aliphatic hydrocarbons which have a different number of branched chains. Aromatic hydrocarbons permeate more rapidly than do the saturated (i.e., paraffinic) hydrocarbons. In an example methyl cyclohexane permeated through the membrane more selectively than did isooctane.
"Microporous Membrane Solvent Extraction," Prasad, R., et al., Separation Science and Technology 22 (2&3) 619-640, 1987 examines the phenomenon of dispersion-free solvent extraction through immobilized aqueous-organic interface in a microporous hydrophobic membrane. Expressly investigated was the use of an organic-organic interface to extract aromatics as exemplified by toluene, from a hydrocarbon feedstock, as exemplified by a mixture of toluene in n-heptane, employing a microporous Celgard 2400 polypropylene membrane to partition the feed from the polar extraction solvent, which in this case was NMP. The toluene selectively permeated through the porous Celgard membrane into the NMP thereby reducing the amount of toluene in the feed (raffinate) while increasing the amount of toluene in the permeate phase (extract).